Electron discharge device



G. B. BANKS ELECTRON DISCHARGE DEVICE Filed Nov. 11, 1936 m M R m NW N NI EWZW VI. mu n H A mw G B Patented June 6, 1939 UNITED STATES PATENT OFFICE ELECTRON DISCHARGE DEVICE George Baldwin Banks,

Chelmsford, England, as-

signor to Radio Corporation of America, a corporation of Delaware Claims.

This invention relates to television and similar transmitting apparatus and more particularly to apparatus for translating Varying values of light and shade in a subject of transmission into elec- 5 trical picture signals.

The principle of the invention is based upon the fact that secondary emission from a metal surface under electron bombardment may be greatly increased by subjecting the said surface to light. For example an aluminium plate which under electron bombardment in the dark gives a secondary emission current of about 308 units may, with the same bombardment, be caused to give an electron emission current of about 869 units by exposing the plate to light. This eflect is apparently not due to the addition of any photo-electric current flow as in the specific case just mentioned the current due to photo-electric emission was only about 40 units.

The present invention consists in utilizing this physical phenomenon for the generation of electrical picture signals for television or like transmission.

According to this invention an electrode, which is subjected to an electron bombardment, is also subjected to light corresponding to the light and shade Values in a subject of transmission and the resultant secondary emission from the electrode, which will vary in amount in dependence upon variation in the light, is utilized to give picture signals.

The invention is illustrated in the accompanying drawing which shows schematically one way of carrying out the invention.

Referring to the drawing there is employed a television cathode ray transmitter tube l of general construction resembling the now Well known iconoscope type of transmitter tube, the principal difference being that the normally pro- 40 vided photo-electrically sensitive mosaic electrode of an iconoscope is replaced by a plain metal sheet electrode 2 of suitable work function. Thus there may be an electron gun 3 of any known construction and arrangement; deflecting plates and/or deflecting coils for deflecting the cathode ray from the gun for scanning action as well known per se (mutually perpendicular pairs 4, 5 of plates are shown); and a plate electrode 2 of aluminium, the plate 2 being approximately rectangular and being positioned to be scanned by the ray, the plane of the plate being at an angle to the normal (undeflected) cathode ray direction (represented by the broken line 6). The tube is provided with a suitable Window 1 opposite the aluminium plate 2 and any convenient optical system represented by a lens 8 is provided for projecting images of a subject of transmission (represented at 9) on to the plate electrode 2 just as, in the iconoscope tube, images of a subject of transmission are projected on to the mosaic electrode. The interior portion of the large end of the tube envelope is silvered over or otherwise coated with conductive material [0. The positive terminal of a suitable source ll of potential is connected at E2 to the accelerating anode of the electron gun, the negative terminal of said source being connected by lead l3 to the cathode of said gun. The said positive terminal is also connected by lead [4 to the conductive deposit ID on the inside of the envelope. A tapping point 5 which may be adjustable upon the source II of potential is connected through a resistance II to the aluminium plate electrode [2. In use the cathode ray from the gun is sharply focused upon the plate electrode 2 and is caused to scan it in any well known way. At the same time images of the subject of transmission are projected through the window 1 upon the same side of the aluminium plate electrode as that which is scanned by the cathode ray.

It will be apparent that with this arrangement the total current flowing through the resistance I6 will include two component currents of opposite sign, one a component current due to primary electrons and the other a component current due to secondary electrons. Preferably the arrangement is such that when the plate electrode is in the dark these two oppositely directed components of current substantially or approximately neutralize one another. Since the secondary electron emission Will Vary with the extent of illumination of the point from which that emission takes place, picture signals will be developed across the resistance I6 as the cathode ray scans the plate. In use the plate 2 is maintained a few volts negative with respect to the metal deposit II] on the inside of the bulb so that disposal of the secondary electrons is facilitated, these electrons being collected by the said metal deposit which, in the illustrated arrangement, is at the same potential as the accelerating anode of the electron gun.

Obviously the invention is not limited to apparatus wherein both co-ordinates of scanning are obtained by movement of a cathode ray; for example, in a tele-cinematograph transmitter the ray could be simply deflected back and forth in a line, the other component of scanning being obtained in the well known way by movement of the film which forms the subject of transmission. Clearly in such an apparatus the electrode which emits secondary electrons can be merely a strip since it need not have more than one picture line image at a time thereon.

Again the invention is not limited to cases where the secondary emissive electrode is subjected to visible light, for obviously the light used for projecting images on to the said electrode could be light of a wave length outside the spectrum of human visibility. The term light is used in this specification, therefore, in:a wide sense to include also (for example) ultra violet light.

Having described my invention, what I claim 1. Apparatus for translating values of light and shade into electrical currents comprising an electrically and physically continuous substantially non-photoelectrically sensitized metallic electrode capable of secondary emission when electronically bombarded, means for subjecting said electrode to electronic bombardment, means for also subjecting said electrode to the light and shade values to be translated so as to alter proportionally the secondary emission properties from unilluminated normal values, and means for utilizing the resultant secondary electron emission from said electrode.

2. A cathode ray tube apparatus for translating a subject of television into picture signals comprising a cathode ray tube having therein a substantially non-photoelectrically sensitized metallic target electrode with a surface capable of secondary electron emission when electronically bombarded and an electron gun for projecting a ray of electrons to bombard said surface, means for subjecting elemental areas of said surface to light activation corresponding to the light and shade values of elemental areas of said subject to cause abnormal secondary emission from the target electrode proportional to the light value of illumination, means for causing said cathode ray to scan said surface, a collector electrode for collecting secondary emission from said surface; and a utilization circuit including said collector and target electrodes.

3. Apparatus as claimed in claim 2 and comprising in addition, means for projecting images of a subject of transmission onto the target electrode which is capable of secondary emission,

means 'for'positively biasing said target electrode relatively to the cathode of the electron gun, means for biasing the collector electrode positively relative to the secondary emitting target electrode, a picture signal output impedance included in circuit with said target and collector electrodes so that the current flow through said impedance varies in dependence upon the secondary emission from the target electrode, and means for causing the cathode ray to scan the secondary emitting electrode.

4. Apparatus as claimed in claim 2 comprising, in addition, means for rendering the current through the utilization circuit substantially zero in the absence of light activation of the secondary emissive target electrode.

5. Apparatus as claimed in claim 2 and wherein the target electrode capable of secondary emission is an aluminium surface.

GEORGE BALDWIN BANKS. 

